The use of the scintillation proximity assay (SPA) to study enzyme binding and interactions is a new type of radioimmunoassay and is well known in the art. The advantage of SPA technology over more conventional radioimmunoassay or ligand-binding assays, is that it eliminates the need to separate unbound ligand from bound ligand prior to ligand measurement. See for example, Nature, Vol, 341, pp. 167-178 entitled "Scintillation Proximity Assay" by N. Bosworth and P. Towers, Anal. Biochem. Vol. 217, pp. 139-147 (1994) entitled "Biotinylated and Cysteine-Modified Peptides as Useful Reagents For Studying the Inhibition of Cathepsin G" by A. M. Brown, et al., Anal. Biochem. Vol. 223, pp. 259-265 (1994) entitled "Direct Measurement of the Binding of RAS to Neurofibromin Using Scintillation Proximity Assay" by R. H. Skinner et al. and Anal. Biochem. Vol. 230, pp. 101-107(1995) entitled "Scintillation Proximity Assay to Measure Binding of Soluble Fibronectin to Antibody-Captured alpha.sub.5 B.sub.1 Integrin" by J. A. Pachter et al.
The basic principle of the assay lies in the use of a solid support containing a scintillation agent, wherein a target enzyme is attached to the support through, e.g., a second enzyme-antienzyme linkage. A known tritiated or I.sup.125 iodinated binding agent, i.e., radioligand inhibitor ligand for the target enzyme is utilized as a control, which when bound to the active site in the target enzyme, is in close proximity to the scintillation agent to induce a scintillation signal, e.g., photon emission, which can be measured by conventional scintillation/radiographic techniques. The unbound tritiated (hot) ligand is too far removed from the scintillation agent to cause an interfering measurable scintillation signal and therefore does not need to be separated, e.g., filtration, as in conventional ligand-binding assays.
The binding of an unknown or potential new ligand (cold, being non-radioactive) can then be determined in a competitive assay versus the known radioligand, by measuring the resulting change in the scintillation signal which will significantly decrease when the unknown ligand also possesses good binding properties.
However, a problem arises when utilizing a target enzyme containing a cysteine group, having a free thiol linkage, --SH, (or present as --S.sup.-) which is in the active site region or is closely associated with the active site and is important for enzyme-ligand binding. If the unknown ligand or mixture, e.g. natural product extracts, human body fluids, cellular fluids, etc. contain reagents which can alkylate, oxidize or chemically interfere with the cysteine thiol group such that normal enzyme-ligand binding is disrupted, then false readings will occur in the assay.
What is needed in the art is a method to circumvent and avoid the problem of cysteine interference in the scintillation proximity assay (SPA) procedure in enzyme binding studies.